Help With Rendezvous

[clok554]

hello, i am extremely frustrated with trying to rendezvous. i get a dectent orbit we're about 1.5 km apart. and then i try to rcs or burn towrds the other craft and it zooms off or i miss entireley! please help i really want to do this.

[kurja]

Lots and lots of threads about rendezvous and docking, if you look around a bit.

If you're already getting close you're almost there already;

-at close distances, do not use your main engines to move closer, use your RCS instead. That allows for much finer control.

-keep the relative speed low, so you don't overshoot

-try switching to docking mode, so you get relative speed and target markers on your navball

Or if the problem is that you can't get closer than 1.5km, remember that to catch up you need to accelerate away from the target so you get into a lower orbit (which is faster). If you accelerate prograde towards the targeat ahead you'll just get pushed into a higher and slower orbit and so you'll just end up farther behind. Start firing (with RCS!) directly towards your target only when you're already within visual range.

If all else fails, search youtube for Scott Manley's videos on the subject =D

[capi3101]

Here are the basics, doesn't matter if you're using LOX thrust or RCS.

1) Make sure the velocity gauge over the navball is set to "Target". If it isn't, you can switch to docking mode or just click it until it comes up.

2) Watch the distance between you and the target; easiest way I find to do this is with the map but the data will come up in the flight view as long as you've got your target selected.

3) If the distance starts to creep back up, aim your ship at the retrograde marker (the yellow one with an x in the center) and reduce your relative velocity to zero m/s.

4) Once your relative speed is zeroed, aim at the target (the purple doughnut with a meatball in the center). Thrust ahead - ten meters per second should be sufficient if you're more than a thousand meters out. You should notice the distance dropping again.

5) Repeat as necessary. Slow your relative velocity as the distance closes - a good rule of thumb is 1 m/s for every 100 meters or so.

6) At 100 meters, slow to zero, adjust your aim and thrust towards the target. If you're aiming for a specific docking port on the target, this would be a good time to target it.

7) At 50 meters, slow to zero, adjust your aim and thrust towards the target.

8) At 20 meters, slow to zero and adjust your aim. Here's where you need to make sure you're aligned for docking. Easiest way to do this is to switch over to the second craft, target the first craft, and aim the second craft towards the first. If you haven't got a lot of torque or its turning response is crappy, it helps to timewarp for an ever so short amount of time once it's aligned.

9) Switch back to the first craft, adjust your aim again and thrust towards the target.

10) At 10 meters, slow to zero, adjust your aim carefully and thrust towards the target. No faster than 0.5 m/s at this pont; 0.3 is probably optimal.

11) At 5 meters, slow to zero, adjust your aim carefully and thrust towards the target.

12) Once the ports get within a meter or so of each other, they'll try and pull each other in. If your alignment isn't spot on, the two craft may dance around a bit; don't panic, just wait.

Let us know how it turns out.

[blizzy78]

hello, i am extremely frustrated with trying to rendezvous. i get a dectent orbit we're about 1.5 km apart. and then i try to rcs or burn towrds the other craft and it zooms off or i miss entireley! please help i really want to do this.

If I'm interpreting this correctly, your problem is not getting into a close distance to your target vessel, but simply slowing down relative to it. Your velocities don't match, which is why it speeds past you (or the other way around.)

There's a simple fix:

- Select the vessel as your target.

- Make the navball show "Target" instead of "Orbit" or "Surface" by clicking on the text in the navball.

- Point your vessel at the retrograde marker on the navball. In navball "Target" mode, this is no longer your orbit retrograde marker, but the marker you should fire your engines to to slow down relative to your target.

- Fire engine and watch "Target XXX m/s" on the navball. That's your relative velocity to your target. You want to bring that down as close to 0 m/s as you can.

- Approach target using RCS or low engine thrust.

- ...

[MalfunctionM1Ke]

Here is a nice Video from Scott which helped me a lot.

It is of 0.18 but physics dont Change i guess

[clok554]

ok thanks for all your help. i assume to slow relative to my target you mean the pink one with the dot i the middle? also i am above my target laterally.(i mean north and south, as i am on a near equator orbit) how do i fix this without skewing my inclination

[Frederf]

Long-duration slow maneuvers you must think of as transfer orbits. Short-duration fast maneuvers are "point and shoot." Outside of 50km think transfer orbits. Inside 10km think point and shoot. The difference between the two regimes is what orbiting around a common body does to your velocity vectors.

At 1.5km you're well inside direct-path range. The goal in this situation is to "put the thing on the thing." The thing in this case is the direction to target and the thing is the velocity vector. Adjusting your velocity vector can be confusing if you're not used to it. In the general case you are going to have two components to your velocity with respect to the target, axial and lateral. Axial is motion toward or away and lateral is sideways.

To travel to the destination we want a manageable positive axial motion (toward it) and minimal lateral motion (sideways). A good first step is to put your nose on the target (pink round) and roll until the velocity vector (yellow, either one) is up. If the anti-velocity vector is visible in the hemisphere around the target direction then you are traveling away from it. Burn toward the target until the prograde velocity vector is in sight. If not, skip that step and proceed to next.

After the previous steps we should be traveling toward the target but also missing it due to some lateral motion. To cancel the lateral motion we point opposite and burn to cancel that component of our velocity without adjusting your toward/away part. As an example we're traveling towardish the target with our nose on it and our velocity vector showing prograde somewhere between the pink-round and the top of the navball. We've got excess "up" lateral velocity that we want to cancel. We do this by pointing 90 degrees "down" and burning a little bit. How much is a matter of some math or guess and check. After several iterations of this the velocity vector and the target market should merge.

Then it's a question of speed and timings. What's your relative speed? Are you going to get there in 200 milliseconds or 20 weeks? Personally I strive for a relative speed that arrives in 100 seconds. It's easy math and comfortable. If something is 20km away I want to have a closing speed of 200 m/s. For 5km, 50 m/s, etc. Orbiting Kerbin happens once every 30 minutes or so. If your rendezvous takes a significant fraction of this time (1/4 orbit or more) then your aiming gets complicated very quickly and it's better to switch your brain to intercept orbital transfer mode.

At 10 seconds I turn around and reduce speed to match the 100 seconds rule again. So 9km, 90m/s. At 900m I slow to 9 m/s. At 90m I slow to 0.9 m/s. Inside 100m I keep docking slow and visual. If you're close in position and speed then the disrupting effects of orbiting are minimized. Once inside 100m and 0.5 m/s you are effectively colocated and static and the fact you're orbiting shouldn't be too bothersome.